a(n) = 5*2^(n-1) + 3*6^n/2.

Terms

a(0) =4a(1) =14a(2) =64a(3) =344a(4) =1984a(5) =11744a(6) =70144a(7) =420224a(8) =2520064a(9) =15117824a(10) =90701824a(11) =544200704a(12) =3265183744a(13) =19591061504a(14) =117546287104a(15) =705277558784